Fuel cells are devices for gaining electric energy by an operational principle based upon reverse action of electrolysis of water. In general, hydrogen gained by transforming fuel such as natural gas, methanol, and coal, and oxygen in the air are supplied to the fuel cells in order to obtain direct-current power while generating water. Because the fuel cells have a high efficiency of electric power generation and are capable of supplying clean energy, fuel cell power generation has attracted attention.
The fuel cells are classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, and a solid polymer type, for example, depending on the type of electrolyte used. In particular, polymer electrolyte fuel cells in which an ion exchange membrane (solid polymer electrolyte membrane) is used as an electrolyte are advantageous in that they are substantially exclusively composed of solid cells and are therefore not subject to the problem of scattering of the electrolytes or the maintenance thereof, the fuel cells can operate at low temperatures of not more than 100° C., the start-up time is extremely brief, and they are capable of achieving high energy density and reduction in size and weight, for example.
Therefore, polymer electrolyte fuel cells are being developed as power sources for automobiles, dispersed-type power sources for homes and buildings, power sources for space vehicles, and portable power sources. Specifically, in terms of environmental issues such as global warming and measures to reduce exhaust gas of automobiles, polymer electrolyte fuel cells are gaining attention as fuel cells to be used for automobiles.
Solid polymer electrolytes constitute a solid polymer material having an electrolyte group such as a sulfonic group in a polymer chain. As the solid polymer electrolytes have the property to strongly bind to specific ions and to allow positive or negative ions to be selectively transmitted, they are formed as particles, fibers, or membranes and are used for various applications such as electrodialysis, diffusion dialysis, and battery diaphragms.
For example, a polymer electrolyte fuel cell comprises a proton-conductive solid polymer electrolyte membrane with a pair of electrodes provided with one electrode on each side thereof. Hydrogen gas gained by reforming low molecular weight hydrocarbon such as methane and methanol is supplied to one of the electrodes (fuel electrode) as fuel gas, and oxygen gas or air is supplied to the other electrode (air electrode) as an oxidant in order to obtain an electromotive force. Water electrolysis is a method for producing hydrogen and oxygen by electrolyzing water using a solid polymer electrolyte membrane.
In consideration of the application of the polymer electrolyte fuel cells to electric automobiles, it is desired that the operation temperature of a fuel cell system be not less than 100° C. for downsizing the cooling system and improving the CO tolerance and the efficiency of the electrode catalyst. At such high temperatures, the vapor pressure of water increases, so that if the internal pressure of the batteries is to exist at a realistic level, the relative humidity of ambient atmosphere declines, making it necessary for the electrolyte membrane to have a sufficient proton conductivity in a low humidity environment.
In addition, although there is a demand that humidification from the outside using pure water be eliminated in order to simplify the system and avoid the problem of freezing in winter, if humidification is eliminated, it would require the ambient atmosphere inside the fuel cells to be maintained in a humid state using only generated water, resulting in low humidity environment likewise.
However, in general, the polymer electrolyte fuel cells are usually operated at a temperature of not more than 100° C. This is because perfluoro electrolyte membranes such as Nafion (registered trademark of DuPont) gains proton conductivity by containing water. Therefore, the water content of the membrane (the water content relative to the weight of dried membrane) is an extremely important factor. The membrane must be maintained in a sufficiently water-containing state if it is to produce proton conductivity, so that water control is required. Consequently, in general, reactant gas must be humidified when operating the batteries. However, humidification of the membrane becomes insufficient at high temperatures of not less than 100° C., so that the proton conductivity declines.
Also, the perfluoro electrolyte membranes are difficult to manufacture, and are highly expensive. This makes it difficult to apply the perfluoro electrolyte membranes to consumer use such as polymer electrolyte fuel cells as low-pollution power sources for automobiles
As mentioned above, perfluoro electrolyte membranes such as Nafion cannot maintain strength at high temperatures or sufficient conductivity in a high temperature and low humidity environment, so that it is difficult to operate fuel cells in high temperature and low humidity conditions. Moreover, the cost is inevitably high.
In order to realize a fuel cell system capable of stable operation under such conditions as high temperature or a lack of humidification, it is extremely important to realize an electrolyte that exhibits conductivity in a low humidity environment. However, there has been no electrolyte having a high ion-exchange capacity and capable of a high degree of dissociation in a low humidity environment while providing a practical strength, and exhibiting sufficient proton conductivity in a high temperature and low humidity environment.
JP Patent Publication (Kokai) No. 7-90111 A (1995) 1 discloses an invention in which a metal catalyst and metal oxide are included in an electrolyte membrane in order to provide a polymer solid electrolyte composition having oxidation resistance equal to or greater than that of a fluorine electrolyte, or sufficient in practical use, which is superior in ion conductivity and the effect of crossover inhibition by having the ability of self generation and retainment of water, and which is most suitable as a membrane for an electrochemical cell such as a polymer solid electrolyte fuel cell. Specifically, at least one metal catalyst selected from the group consisting of platinum, gold, palladium, rubidium, iridium, and ruthenium is included in a polymer solid electrolyte selected from the group consisting of cation exchange resins and/or anion exchange resins, and microscopic particles and/or fibers of metal oxide such as silica and titania are further included.